1. Field of Invention
The present invention relates to a method for the combustion of coal wherein the emissions of SO.sub.2 are reduced.
2. Description of the Prior Art
In the combustion of carbonaceous materials such as coal which contains sulfur and ash, oxygen may combine with the sulfur to produce sulfur dioxide. Production of sulfur dioxide is undesirable. Government regulations limit the amount of sulfur dioxide which may be emitted from a combustion furnace. To comply with these regulations, utilities generally have elected to use low sulfur coals or to use alternate fuels such as oil and gas or to use expensive scrubbers. Low sulfur coals may be more expensive than coals with higher sulfur content or may incur logistic and/or transport expense. Because of this price difference, numerous attempts have been made to develop processes for burning coals of higher sulfur content without producing increased emission of sulfur dioxide.
The art has pursued at least two methods of burning coal to reduce sulfur emissions. One process involves the addition of a reagent, such as limestone, to the coal. In many furnaces, coal is pulverized and injected into the combustion chamber in powder form. Prior to, during or after the injection of coal into a furnace, limestone or other reagents are mixed with the coal. The reagent provides a material, such as calcium oxide, which will combine with sulfur dioxide formed during combustion. In that way emission of sulfur dioxide is reduced.
A second method is simply to dilute the coal with another fuel that contains no sulfur. One example would be to inject gas or low sulfur oil into the combustion chamber along with powdered coal. It has generally been believed that the reduction in sulfur dioxide emissions in the flue gases would be proportional to the reduction in overall percentage of sulfur content of the combined fuels. If a coal containing 0.5 percent sulfur were combined with natural gas that contains no sulfur to form a fuel that is 90 percent coal and 10 percent gas, the sulfur content of the resulting fuel would be 0.45 percent based on the heat of combustion. This method has generally not been followed because coal prices are substantially less than the prices of gas and oil. Thus, there is little cost benefit in combining these fuels to significantly reduce sulfur dioxide emissions.
There have also been numerous methods proposed for removing sulfur dioxide from the gases escaping from the combustion process. The most common commercial practice is to scrub the flue gas with lime or limestone sprays or solutions which effectively removes the sulfur dioxide. This scrubbing process is very expensive.
All of these prior art methods have disadvantages. A principal problem is that most coal furnaces which are now in operation are not designed to accommodate any of these techniques, and major modifications are required to utilize these methods. Such retrofitting is expensive. Consequently, there is a need for a coal combustion process which will reduce sulfur dioxide emissions and which can be readily used in existing coal furnaces.
The use of reagents, as well as substitution of alternate fossil fuels, increases the costs of the combustion process. Unless these increases can be offset with the use of low cost, high sulfur coal, these methods increase the cost of power generation. Accordingly, there is a need for a process that will enable one to burn low cost, higher sulfur, non-compliance fuels and provide a net savings over conventional methods.